A helicopter is generally provided with at least two turboshaft engines which operate at speeds that depend on the flight conditions of the helicopter. Throughout the following text, a helicopter is said to be in a cruise flight situation when it is progressing in normal conditions, during all the phases of the flight apart from transitional phases of take-off, ascent, landing or hovering flight. Throughout the following text, a helicopter is said to be in a critical flight situation when it is necessary for it to have available the total installed power, i.e. during the transitional phases of take-off, ascent, landing and the mode in which one of the turboshaft engines is malfunctioning, referred to by the abbreviation OEI (One Engine Inoperative).
It is known that when the helicopter is in a cruise flight situation, the turboshaft engines operate at low power levels, below their maximum continuous power (hereinafter MCT). In some configurations (forward speeds of less than the maximum speed, the helicopter not flying at the maximum mass, etc.), the power provided by the turboshaft engines during a cruise flight can be less than 50% of the maximum take-off power (hereinafter MTO). These low power levels result in a specific consumption (hereinafter SC), which is defined as the relationship between the hourly fuel consumption by the combustion chamber of the turboshaft engine and the mechanical power provided by said turboshaft engine, of greater than approximately 30% than the SC of the MTO, and thus in an overconsumption of fuel during cruise flight.
Finally, during holding phases on the ground, pilots generally prefer to put the various turboshaft engines into ground idling so as to be certain of being able to restart them. The turboshaft engines thus continue to consume fuel, despite not providing any power.
At the same time, the turboshaft engines are also oversized so as to be able to ensure flight over the entire flight range specified by the aircraft manufacturer, and in particular flight at high altitudes and during hot weather. These flight points, which are very restrictive, in particular when the helicopter has a mass that is close to its maximum take-off mass, are only encountered in specific use cases of some helicopters. As a result, although dimensioned so as to be able to provide such powers, some turboshaft engines will never fly in such conditions.
These oversized turboshaft engines are disadvantageous in terms of mass and fuel consumption. In order to reduce this consumption during cruise flight or during holding on the ground, it is possible to stop one of the turboshaft engines and to put it into a mode referred to as standby mode. The active engine or engines then operate at higher power levels in order to provide all the necessary power, and therefore at more favourable SC levels. However, this practice is contrary to the current certification rules, and turboshaft engines are not designed to ensure a level of restart reliability that is compatible with safety standards. Likewise, the pilots are not currently aware of or familiar with the idea of putting a turboshaft engine into standby mode during flight.
As is known, a turboshaft engine of a helicopter comprises a gas generator and a free turbine which is powered by the gas generator in order to provide power. The gas generator is conventionally made up of air compressors which are connected to a chamber for combusting the fuel in the compressed air, which chamber supplies hot gases to turbines for partially expanding gas, which turbines rotate the compressors by means of drive shafts. The gases then drive the free power transmission turbine. The free turbine transmits power to the rotor of the helicopter by means of a gearbox.
In FR1151717 and FR1359766, the applicants proposed methods for optimising the specific consumption of the turboshaft engines of a helicopter by the possibility of putting at least one turboshaft engine into a stable flight mode, referred to as continuous flight mode, and at least one turboshaft engine into a particular standby mode that it can leave in an emergency or in a normal manner, according to need. A transition out of standby mode is referred to as ‘normal’ when a change in the flight situation requires the turboshaft engine in standby to be activated, for example when the helicopter is going to transition from a cruise flight situation to a landing phase. A normal transition out of standby mode of this kind occurs over a period of between 10 seconds and 1 minute. A transition out of standby mode is referred to as ‘emergency’ when there is a failure or a power deficit in the active engine, or when the flight conditions suddenly become difficult. An emergency transition out of standby mode of this kind occurs over a period of less than 10 seconds.
The applicants have proposed in particular the following two standby modes:                a standby mode referred to as normal super-idling, in which the combustion chamber is ignited and the shaft of the gas generator rotates at a speed of between 20 and 60% of the nominal speed,        a standby mode referred to as assisted super-idling, in which the combustion chamber is ignited and the shaft of the gas generator rotates, with mechanical assistance, at a speed of between 20 and 60% of the nominal speed.        
A disadvantage of the normal super-idling mode is the operating temperatures, which become increasingly high as attempts are made to reach ever lower idling.
The assisted super-idling mode makes it possible to remedy this problem of operating temperature. However, this requires the use of an electrical or pneumatic drive machine and of a corresponding coupling.
In addition, the technical problem now arises of achieving a super-idling mode which is not mechanically assisted but which is not limited by the temperatures of the turboshaft engine. The technical problem addressed is therefore that of providing a turboshaft engine that makes it possible to provide an improved super-idling mode of this kind.